1. Field of the Invention
The present invention relates to optical imaging devices, called objectives, intended for example to equip picture-taking devices, called cameras, and providing images over an extended spectral range ("white light").
2. Discussion of the Background
In the current state of the art, these objectives consist of a succession of centred spherical lenses, composed of various materials which are transparent in the spectral range in question, for example made of glass for the visible wavelengths or made of germanium, silicon or other metalloids for the near-infrared range.
According to methods well known to those skilled in the art, these various lenses, of which these imaging objectives are composed, are matched and made from materials having dispersive properties chosen for correcting as far as possible the aberrations from the so-called longitudinal chromatism, which results from a variation in the focal length of the objective with wavelength. This thus produces what is called aliasing of the spectrum.
Again, methods well known to those skilled in the art are also used to correct aperture aberration, which is on the axis the spherical aberration, in three ways: either by combining a sufficient number of spherical lenses, or by the use of one or more aspheric surfaces, or by the use of gradient index materials. Thus, a correction is made for a central wavelength.
However, a drawback arises in the form of a significant and spurious variation in the spherical aberration with wavelength or, which amounts to the same thing, of the chromatic aberration with the aperture, which phenomenon constitutes spherochromatism.
Thus, for an infrared objective composed of a silicon-germanium doublet, the construction parameters of which are given in Table 1 below:
TABLE 1 ______________________________________ Radius of curvature (mm) Thickness (mm) Material ______________________________________ +91.95 5 Si +287.5 2 +375.1 2 Ge +175.4 ______________________________________
the longitudinal spherochromatic aberration curves, shown in FIG. 1, highlight this phenomenon. In these curves, the horizontal axis corresponds to the optical axis of the lens and the vertical axis to an axis perpendicular to the optical axis of the lens. Along this vertical axis is plotted the value of the radius of the pupil which is placed in front of the doublet in the direction of the optical path. For a radius of the pupil equal to 0, the curve gives the value of the residual longitudinal chromatism, i.e. 7 microns in the case of the example chosen. For a certain radius of the pupil, a maximum aberration is obtained, i.e. a residual spherical aberration of 17 microns in FIG. 1. Finally, for a maximum radius of the pupil, i.e. at full aperture, it is observed that the longest, 5 micron wavelength radiation is precisely corrected for spherical aberration, as is the 4 micron wavelength central radiation. However, at this full aperture, the shortest radiation, of 3 micron wavelength, is affected by a 20 micron longitudinal aberration responsible for the perceptible drop in performance of the objective at this full aperture.
Aspherization processes, involving the removal of material (for example, machining or ion bombardment) or involving the addition of material (vacuum deposition of a coating of variable thickness, according to U.S. Pat. No. 7,806,422, filed on Mar. 7 1978 by the Applicant) are known, but they do not apply in the case of chromatism.
Processes for obtaining gradient index materials, having a radial or longitudinal index gradient, are also known, for example ion-diffusion or melting processes.
However, these materials have certain drawbacks. A first drawback with these gradient index materials, in the case of the ion-diffusion process, resides in the fact that the number of materials capable of being diffused in this way is limited, because of physico-chemical compatibility constraints.
A second drawback with these materials, in the case of the process involving successively melting a stack of various glasses, is that there are thermal expansion constraints which preclude a choice of glasses of different chemical groups, which practically removes any useful influence on the correction of chromaticity aberrations.